Connectors for electrically interconnecting circuit boards in a face-to-face relation (board-to-board connectors) are traditionally being used. FIGS. 1 and 2 illustrate a configuration described in Japanese Patent Application Laid Open No. 2012-252785 (published on Dec. 20, 2012, hereinafter referred to as Literature 1), which is an example of such conventional connectors. Illustrated in FIGS. 1 and 2 are a connector (receptacle connector) 10 designed to be mounted on a circuit board, not depicted, and a mating connector (plug connector) 20 designed to be mounted on another circuit board, not depicted, and to be mated with the connector 10. FIG. 2 illustrates a cross-sectional structure of the connector 10 and the mating connector 20 mated together.
The connector 10 includes a housing 11, contacts 12a, 12b held by the housing 11, and a pair of shells 13a, 13b which are provided on sidewall parts of the housing 11 so as to surround the housing 11 and the contacts 12a, 12b, and are designed to be grounded to a circuit board, not depicted.
The mating connector 20 includes a main body 21, a pair of mating fitting parts 22a, 22b protruding from the lower surface of the main body 21 in parallel with each other, a plurality of mating contacts 23a, 23b which are provided along the length of the mating fitting parts 22a, 22b at a predetermined pitch and are designed to be connected to the contacts 12a, 12b of the connector 10, and ground terminals 24a, 24b which are provided at both ends of the mating fitting parts 22a, 22b and are designed to be connected to the shells 13a, 13b. 
The housing 11 of the connector 10 has its length along the pitch direction of the contacts 12a, 12b and includes a pair of opposed first sidewall parts 14a, 14b and a pair of opposed second sidewall parts 14c, 14d which join the ends of the pair of first sidewall parts 14a, 14b. 
Grooves 15a, 15b in which the mating fitting parts 22a, 22b of the mating connector 20 are to be inserted are provided in the upper surface of the housing 11. Contact insertion holes 16a, 16b are provided across the grooves 15a, 15b at a predetermined pitch along the length of the housing 11. The ends of the grooves 15a, 15b are joined by joining grooves 17a, 17b and a block-like raised part 18 is formed in the center.
As illustrated in FIG. 3, the shell 13a includes a joining main part 31 disposed along the second sidewall part 14c of the housing 11, a short-side main part 32 disposed along the first sidewall part 14b, and a long-side main part 33 disposed along the first sidewall part 14a. The joining main part 31 has a bent part 34 and a shell ground part 35 which protrudes from the bent part 34 toward the inner side of the shell 13a and is designed to contact the ground terminal 24a of the mating connector 20 is provided.
A short-side latch part 32a is provided at the short-side main part 32 for engagement and securing of the shell 13a to the housing 11 and connection with a ground terminal on a circuit board. An end latch part 33a and a long-side latch part 33c are provided at the long-side main part 33 for connection with a ground terminal on the circuit board. A long-side latch part 33b is further provided at the long-side main part 33 for engagement and securing of the shell 13a to the housing. Note that the shell 13b has the same shape as the shell 13a. 
The connector 10 is assembled by pressing the contacts 12a, 12b into the contact insertion holes 16a, 16b, respectively, of the housing 11 and engaging and securing the above-mentioned latches of the shells 13a, 13b to press-fit parts provided in the sidewall parts of the housing 11.
The mating of the connector 10 with the mating connector 20 is accomplished by inserting the mating fitting parts 22a, 22b of the mating connector 20 into the grooves 15a, 15b, respectively, of the connector 10. As a result, the mating contacts 23a, 23b of the mating connector 20 are pinched by the contacts 12a, 12b, respectively, of the connector 10, thereby the connectors 10 and 20 are interconnected.
As illustrated in FIG. 2, the ground terminals 24a, 24b of the mating connector 20, on the other hand, are inserted in the joining grooves 17a, 17b of the connector 10 and are brought into contact with the shell ground parts 35 of the shells 13a, 13b and are thereby grounded.
In this way, the mating connector 20 is mated and electrically connected with the connector 10.
FIG. 4 illustrates another exemplary shape of a shell described in Literature 1. The shell 13c illustrated in FIG. 4 has a shell ground part 35 provided on the inner side of a long-side main part 33.
There is a demand for size reduction of connectors of this type (board-to-board connectors) in order to save mounting space and, with the decrease in spacing between circuit boards to interconnect, there is also a growing demand for reduction in the profile of connectors.
In these circumstances, connectors (receptacle connectors) such as the connector 10 described above that include shells on which spring pieces as ground parts that contact ground terminals of a mating connector are formed require the spring pieces to be highly pliant, reliable and durable. However, spring pieces have reached the limits of thickness and width. Furthermore, increasing the length of spring pieces inhibits reductions of size and profile of connectors. Therefore, it has been difficult to provide highly pliant, reliable and durable spring pieces.